Temperature extremes and especially changes in humidity effect the resistively of transfer components in an electrophotographic (EP) printing system. With increasing humidity, resistivities of transfer rolls, intermediate belts, and paper all decrease. When the resistively decreases, it is more difficult to build a transfer field to move toner from an intermediate belt to the print media (e.g., paper).
Toner that is printed onto an intermediate belt or drum will gain charge when passing under additional color nips, such as found in a color laser (or EP) printer that has multiple color printing stations, in which each printing station has a transfer nip comprised of a photoconductive drum and a transfer roller. The gain in charge by passing under additional color nips can frequently improve the print quality of the toner by improving the “force” on the toner, thereby encouraging it to transfer. (In the case, the word “force” is equivalent to voltage or “EMF” {electromotive force}.) However, the last nip in the process sequence, typically the black tone station, does not benefit from charge boost from additional nips. To compensate for this, some printer designers have added coronas or other charge members to increase the toner charge before transfer. These additional members can have environmental or cleaning issues, and also have associated additional costs. It may be difficult to justify adding such additional members when the problem manifests itself only during high humidity conditions.